Low leakage current offset-gate thin film transistor structure

ABSTRACT

A thin film transistor structure and methods of manufacture provide high ON/OFF current ratio and significantly reduce OFF state leakage currents. A doped thin film disposed on an insulating substrate is etched to form opposing source and drain regions. An undoped thin film is disposed between the opposing source and drain regions so that there is some overlap of the undoped thin film onto the top sides of the source and drain regions. Conventional photomasking, etching and ion implantation steps are then used to form a gate electrode offset from at least the drain region, and preferably offset from both source and drain regions, as well as conventional insulation and interconnect layers. The reduction in electric field intensity in the drain region, and the reduction in trap state density result from, performing heavy junction doping prior to deposition of the undoped thin film, and offsetting the gate electrode from the drain region. This structure provides very low OFF state leakage current while not seriously affecting the ON current. Several alternative fabrication processes are disclosed.

BACKGROUND OF THE INVENTION

The present invention relates to a thin film transistor which isparticularly suitable for use in applications such as active matrixliquid crystal devices, image sensors, or three-dimensional integratedcircuits.

Early thin film transistors had undesirably high OFF state leakagecurrents. One approach to overcoming this problem involved lightly dopeddrain structures, while another approach involved offset-gatestructures. The following section details the structure and manufactureof previous thin film transistors and highlights the inherent problemswhich are overcome by the present invention.

DESCRIPTION OF RELATED ART

An example of the production and structure of a conventional thin filmtransistor (TFT) is explained with reference to FIG. 2, which shows across-sectional view of a conventional TFT structure.

Source and drain areas, or regions, 202, 203, comprised of a siliconthin film, such as polycrystalline silicon (poly-Si) or amorphoussilicon (a-Si), into which a donor or acceptor impurity is added, areformed on an insulating substrate 201 such as glass, quartz, orsapphire. Donor type impurity atoms, such as arsenic and phosphorous,form N-type regions. Acceptor type impurity atoms such as boron formP-type regions. The selection of N-type or P-type dopants is a matter ofdesign choice well understood by those skilled in the art.

Channel area 204, comprised of a silicon thin film, such as poly-Si ora-Si, is disposed between the source and drain regions so as to be incontact with a portion of the top sides of both source and drain regions202, 203. Source and drain electrodes 205, 206 are disposed so as toelectrically contact the source and drain regions respectively.Electrodes 205, 206 may be made from metal, a transparentelectrically-conductive film such as indium-tin-oxide (ITO), or asimilar material. This structure is coated with a gate insulation layer207. Gate electrode 208 is disposed in such a manner as to cover bothsource and drain regions 202, 203, or at least cover a part of each ofthem. Gate insulation layer 207 also serves as an interlayer insulationfor interconnect materials.

The above-described prior art TFT structure has problems however, whichare described with reference to FIG. 3. A graph is presented in FIG. 3,showing typical drain current versus gate voltage characteristics of anN-channel TFT having the conventional structure shown in FIG. 2. Thehorizontal axis indicates gate voltage Vgs, while the vertical axisindicates logarithm values of drain current Id.

In this example, a current that flows between the source and the drainwhen a transistor is in the OFF state, is called the OFF current; acurrent that flows between the source and the drain when a transistor isin the ON state, is called the ON current. A transistor havingcharacteristics such that the ON current is large and the OFF current issmall, or in other words, a high ON/OFF current ratio, is preferable.Large ON current capability provides for fast circuits while very smallOFF currents provide for low power operation. Generally, ON and OFFcurrents tend to track with process variations, so if the ON current isincreased, the OFF current has a tendency to increase. This fact poses aproblem, particularly when an attempt is made to implement anintegrated-driver liquid crystal device. That is, transistors used inthe pixel section of a liquid crystal device are required to have lowOFF current characteristics, whereas transistors used in the peripheralcircuits are required to have high ON current characteristics in orderto operate at high speed.

As can be seen from the drain current versus gate voltage curve of aconventional TFT shown in FIG. 3, the OFF current is a function of gatevoltage, and more particularly, a function of the voltage between thegate and drain. In the case where no threshold adjusting implant hasbeen performed in the channel region, the OFF current is minimum whenVgs is approximately zero. In other words, as the gate to drain electricfield intensity is reduced, the OFF current will also be reduced.

A method of reducing the electric field intensity between the gate anddrain involving lowering the impurity concentration of the drain isknown. This method is explained with reference to FIGS. 4(a)-4(c).

FIG. 4(a) shows a pattern 402 comprised of a silicon thin film, such aspoly-Si or a-Si formed on an insulating substrate 401 of glass, quartz,or sapphire. Next, this structure is coated with gate insulation layer403 comprised of a material such as a silicon oxide film. Formed thereonis a gate electrode 404 which may be comprised of a metal, a transparentelectrically-conductive film (e.g. ITO), an impurity doped poly-Si film,or the like.

Next, as shown in FIG. 4(b), by ion implanting, approximately 1×10¹⁴cm⁻² of donor or acceptor impurities, low-concentration source and drainregions 405, 406 are formed self-alignedly with respect to gate eletrode404

FIG. 4(c) shows that after insulation layer 407 is formed on all of theabove, this insulation layer 407 is anisotropically etched so thatinsulation layer 407 remains only on the side walls of gate electrode404. That portion of silicon oxide film 407 that remains on the sidewalls of the gate becomes a barrier to subsequent ion implantation. Byion implanting, for example, approximately 1×10¹⁵ cm⁻² of donor oracceptor impurities, source and drain regions 408, 409 areself-alignedly formed. Thus a transistor having a low impurity, ordoping, concentration in the drain region is formed. This is sometimesreferred to as a lightly doped drain (LDD) structure.

As shown in FIG. 4(d), source and drain electrodes 410, 411 areconnected to source and drain regions 408, 409 respectively by means ofwell understood conventional processing steps.

However, there are problems associated with this device structure andmethod of manufacture. First, because the drain is formed by ionimplantation, crystal damage is sustained, leading to a higher trapstate density in the drain section of the silicon thin film. The trapstate density in the drain is a major parameter affecting the OFFcurrent. Second, the number of process steps, such as ion implantation,increases.

What is desirable is a thin film transistor structure which has very lowsubthreshold conduction (OFF) currents, a high ON-OFF current ratiocompared to conventional TFTs, and which is also easy to manufacture.

SUMMARY OF THE INVENTION

It is an object of the present invention is to provide a thin filmtransistor having a high ON/OFF current ratio.

It is a further object of the present invention to provide a TFTstructure which has very low leakage, or subthreshold conduction,currents.

Accordingly, the thin film transistor of the present invention ischaracterized by reduced lateral electric field intensity in the drainregion and reduced implant-generated crystal damage in the drain regionwhen compared to conventional TFTs. These characteristic featuresadvantageously provide for a high ON/OFF current ratio, while providingvery low OFF currents.

An offset-gate structure accounts for the reduction in electric fieldstrength in the drain region. Doping the source and drain regions priorto the deposition of the thin film which forms the channel region meansthat the later deposited film experiences only a light doping andtherefore less crystal damage and fewer trap states. The combination ofreduced electric field and reduced trap state density leads to a verylow OFF current while not significantly affecting the ON current. Thereason the ON current is not significantly degraded is that the channelsection silicon layer is thin. This means the range in which a depletionlayer extends is limited and an inversion layer is likely to be formed.Therefore, if the length of the offset section is optimized, a decreasein the ON current can be suppressed. As a result, it has become possibleto provide a thin film transistor having a large ON/OFF ratio. Thisoptimized length is determined empirically by fabricating a large numberof TFTs having different offset lengths, and measuring their electricalcharacteristics to find the offset lengths which give the bestperformance.

An advantage of the present invention is that, a considerableimprovement in the performance of liquid crystal displays and areduction in the costs thereof can be achieved by replacing conventionalTFTs, with TFTs of the present invention. For example, in a conventionalliquid crystal display, since the OFF current of a TFT used in the pixelsection is undesirably large, transistors are connected in series inorder to reduce the OFF current. However, because this does not have tobe done if the TFT of the present invention is used, the yield and thepicture quality of the display can be enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing an example structure of the TFTof the present invention.

FIG. 2 is a cross-sectional view showing an example structure of theconventional TFT.

FIG. 3 is a graph showing a drain current versus gate voltage curve fora conventional TFT.

FIGS. 4(a) to 4(d) are cross-sectional views illustrating the structureof a TFT which uses the known LDD method of reducing OFF current.

FIG. 5 is a graph showing drain current versus gate voltagecharacteristics of the TFT of the present invention.

FIGS. 6(a) to 6(c), FIGS. 7(a) to 7(c), FIGS. 8(a) to 8(c), FIGS. 9(a)to 9(c), FIGS. 10(a) to 10(d), FIGS. 11(a) to 11(d), FIGS. 12(a) to12(d), FIGS. 13(a) to 13(d), FIGS. 14(a) to 14(c), and FIGS. 15(a) to15(c) are cross-sectional views illustrating intermediate manufacturingsteps in which a TFT of the present invention is fabricated.

FIG. 16 is a cross-sectional view showing the structure of oneembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The thin film transistor structure of the present invention, and severalmethods of manufacturing the same, are now described with reference tothe accompanying drawings.

FIG. 1 is a cross-sectional view showing the structure of a thin filmtransistor according to the present invention. Opposing source and drainareas 102, 103 which are comprised of a silicon thin film, such aspoly-Si or a-Si, in which an impurity is added, are formed on aninsulating substrate 101 of glass, quartz, sapphire or the like. As willbe understood by those skilled in the art, the impurity can be either adonor or acceptor type depending on whether a P-channel or N-channeltransistor is to be made. Channel area 104 is comprised of a siliconthin film, such as poly-Si or a-Si, disposed so as to be in contact withsource and drain areas 102,103 in such a manner as to connect the twoareas. Source and drain electrodes 105,106 are disposed so as to be incontact with source and drain areas 102,103 respectively. Source anddrain electrodes 105,106 may be comprised of a metal, a transparentelectrically-conductive film (e.g. ITO), or the like. Theabove-described structure is coated with gate insulation layer 107,comprised of a material such as a silicon oxide. Gate electrode 108which may be comprised of a metal, a transparent electrically-conductivefilm (e.g. ITO), or the like, is disposed in such a manner as not tocover (i.e. overlap) either source or drain areas 102,103, or at leastnot to cover drain area 103. Gate insulation layer 107 also acts as aninterlayer insulation layer for maintaining electrical separationbetween interconnect layers.

The TFT structure of the present invention can be fabricated by means ofseveral alternative processes. Ten example processes are describedbelow.

PROCESS EXAMPLE #1

The TFT of the present invention can be fabricated with the processsteps illustrated by the cross-sectional views of FIGS. 6(a)-6(c).

FIG. 6(a) shows patterns 602,603 which are comprised of a silicon thinfilm, such as poly-Si or a-Si, formed on an insulating substrate 601 ofglass, quartz, sapphire or the like. Pattern 604 comprised of a siliconthin film, such as poly-Si or a-Si, is disposed so as to be in contactwith the top side of the two pattern areas 602, 603 in such a manner asto connect the two areas. Next, all of the above is coated with gateinsulation layer 605 which may be comprised of an insulator such as asilicon oxide film, or the like. Formed thereon is gate electrode 606,comprised of a metal, a transparent electrically conductive film (e.g.ITO), or a poly-Si film.

Next, as shown in FIG. 6(b), an insulation layer 607 is formed on all ofthe above. Insulation layer 607 is preferably a silicon oxide film. Thatportion of film 607 formed on the side walls of the gate is effectivelya thick film when seen from a vertical direction and serves as a barrierfor subsequent ion implantation. By ion implanting impurities (donor oracceptor depending on whether N-channel or P-channel devices aredesired), source and drain areas 608, 609 are formed self-alignedly withgate electrode 606. As a result, a transistor having an offset gatestructure is formed.

Thereafter, as shown in FIG. 6(c), source and drain electrodes 610, 611which may be comprised of a metal, a transparent electrically-conductivefilm (e.g. ITO), or the like, are connected to source and drain areas608, 609 respectively by means of the usual process steps as would beunderstood by those skilled in this art. Thus, a TFT of the presentinvention is completed.

PROCESS EXAMPLE #2

The TFT of the present invention can be fabricated with the processsteps illustrated by the cross-sectional views of FIGS. 7(a)-7(c).

FIG. 7(a) shows patterns 702, 703 which are comprised of a silicon thinfilm, such as poly-Si or a-Si, formed on an insulating substrate 701 ofglass, quartz, sapphire or the like. Pattern 704, comprised of a siliconthin film, such as poly-Si or a-Si, is disposed so as to be in contactwith the top side of the two pattern areas 702, 703 in such a manner asto connect the two areas. Next, all of the above is coated with a gateinsulation layer 705, which may be comprised of an insulator such as asilicon oxide film, or the like. Formed thereon is gate electrode 706,comprised of a metal, a transparent electrically-conductive film (e.g.ITO), or a poly-Si film.

As shown in FIG. 7(b), after formation of insulation layer 707 on all ofthe above, anisotropic etching is performed so that insulation layer 707remains only on the side walls of gate electrode 706. Insulation layer707 is preferably a silicon oxide film. That portion of silicon oxidefilm 707 that remains on the side walls of the gate after etching, actsas a barrier to subsequent ion implantation. By ion implantingimpurities (donor or acceptor depending on whether N-channel orP-channel devices are desired) source and drain areas 708, 709 areformed self-alignedly with gate electrode 706. As a result, a transistorhaving an offset gate structure is formed.

Thereafter, as shown in FIG. 7(c), source and drain electrodes 710, 711which may be comprised of a metal, a transparent electrically-conductivefilm (e.g. ITO), or the like, are connected to source and drain areas708, 709 respectively according to the usual process steps as would beunderstood by those skilled in this art. Thus, a TFT of the presentinvention is completed.

PROCESS EXAMPLE #3

The TFT of the present invention can be fabricated with the processsteps illustrated by the cross-sectional views of FIGS. 8(a)-8(c).

FIG. 8(a) shows patterns 802, 803 which are comprised of a silicon thinfilm, such as poly-Si or a-Si, formed on an insulating substrate 801 ofglass, quartz, sapphire or the like. Pattern 804 comprised of a siliconthin film, such as poly-Si or a-Si, is disposed so as to be in contactwith the top side of the two pattern areas 802, 803 in such a manner asto connect the two areas. Next, formed thereon, are a gate insulationlayer 805 preferably comprised of a silicon oxide film and anelectrically conductive film 806 which will eventually serve as a gateelectrode. Next, as shown in FIG. 8(b) a photoresist pattern 807 isformed on the electrically conductive film 806 by using a standardphotomasking technique. With this pattern as an etch mask, theelectrically conductive film 806 is selectively etched in such a waythat it is small relative to the photoresist pattern (i.e. laterallyoveretched), thus forming gate electrode 808. That is, etching continuesuntil the gate electrode material is undercut with respect to theoverlying photoresist. In this way the overhanging portions ofphotoresist pattern 807 are able to act as an ion implant barrier andfacilitate source/drain formation that is aligned to the photoresist andnot the gate electrode. Ion implantation of impurity atoms (donor oracceptor as described above) forms source and drain areas 809, 810self-aligned to photoresist pattern 807. Photoresist pattern 807 is thenremoved.

Thereafter, as shown in FIG. 8(c), source and drain electrodes 811, 812which are comprised of a metal, a transparent electrically-conductivefilm (e.g. ITO), or the like, are connected to source and drain areas809, 810 respectively, according to the usual process steps as would beunderstood by those skilled in this art. Thus, a TFT of the presentinvention is completed.

PROCESS EXAMPLE #4

The TFT of the present invention can be fabricated with the processsteps illustrated by the cross-sectional views of FIGS. 9(a)-9(c).

FIG. 9(a) shows patterns 902, 903 which are comprised of a silicon thinfilm, such as poly-Si or a-Si, formed on an insulating substrate 901 ofglass, quartz, sapphire or the like. Pattern 904 comprised of a siliconthin film, such as poly-Si or a-Si, is disposed so as to be in contactwith the top side of the two pattern areas 902, 903 in such a manner asto connect the two areas. Next, formed in turn thereon, are gateinsulation layer 905 comprised of an insulator, such as silicon oxidefilm, and an electrically conductive film 906 which will eventuallyserve as a gate electrode.

Next, as shown in FIG. 9(b), a photoresist pattern 907 is formed onelectrically conductive film 906 by using standard photomaskingtechniques. With this pattern as a mask, electrically conductive film906 is selectively etched, and a gate electrode, the lateral dimensionsof which approximate those of the overlying photoresist, is formed. Thisis followed by ion implantation of (donor or acceptor as describedabove) impurity atoms, thereby forming source and drain areas 909, 910which are self-aligned with respect to both the gate electrode and theoverlying photoresist. The gate electrode is then etched so that it issmall relative to the overlying photoresist pattern 907 to form gateelectrode 908. Photoresist pattern 907 is then removed.

Thereafter, as shown in FIG. 9(c), source and drain electrodes 911, 912which are comprised of a metal, a transparent electrically-conductivefilm (e.g. ITO), or the like, are connected to the source and drainareas 909, 910 respectively, according to the usual process steps aswould be understood by those skilled in this art. Thus, a TFT of thepresent invention is completed.

PROCESS EXAMPLE #5

The TFT of the present invention can be fabricated with the processsteps illustrated by the cross-sectional views of FIGS. 10(a)-10(d).

FIG. 10(a) shows patterns 1002, 1003 which are comprised of a siliconthin film, such as poly-Si or a-Si, formed on an insulating substrate1001 of glass, quartz, sapphire or the like. Pattern 1004 comprised of asilicon thin film, such as poly-Si or a-Si, is disposed so as to be incontact with the top side of the two pattern areas 1002, 1003 so as toconnect the two areas. Next, formed in turn thereon are gate insulationlayer 1005 comprised of an insulator such as a silicon oxide film,electrically conductive film 1006 which will eventually serve as a gateelectrode, and silicon oxide film 1007.

Next, as shown in FIG. 10(b), photoresist pattern 1008 is formed onsilicon oxide film 1007 by using a standard photomasking technique. Withthis pattern as a mask, the silicon oxide film 1007 is selectivelyetched to form patterned, post-etch silicon oxide film 1007A.

Thereafter, as shown in FIG. 10(c), photoresist pattern 1008 is removed.Then, with post-etch silicon oxide film 1007A as a mask, electricallyconductive film 1006 is selectively etched so that it is small relativeto post-etch silicon oxide film 1007A. That is, etching continues untilthe gate electrode material is undercut with respect to the overlyingoxide. In this way the overhanging portions of oxide 1007A are able toact as an ion implant barrier and facilitate source/drain formation thatis aligned to the oxide and not the gate electrode. Gate electrode 1009is formed in this way. Ion implantation of impurity atoms (donor oracceptor as described above) forms source and drain areas 1010, 1011self-aligned to oxide 1007A.

Thereafter, as shown in FIG. 10(d), source and drain electrodes 1012,1013 which are comprised of a metal, a transparentelectrically-conductive film (e.g. ITO), or the like, are connected tosource and drain areas 1010, 1011 respectively, according to the usualprocess steps as would be understood by those skilled in this art. Thus,a TFT of the present invention is completed.

PROCESS EXAMPLE #6

The TFT of the present invention can be fabricated with the processsteps illustrated by the cross-sectional views of FIGS. 11(a)-11(d).

FIG. 11(a) shows patterns 1102, 1103 which are comprised of a siliconthin film, such as poly-Si or a-Si, formed on an insulating substrate1101 of glass, quartz, sapphire or the like. Pattern 1104 comprised of asilicon thin film, such as poly-Si or a-Si, is disposed so as to be incontact with the top side of the two pattern areas 1102, 1103 so as toconnect the two areas. Next, formed in turn on all of the above, aregate insulation layer 1105 comprised of an insulator such as a siliconoxide film, electrically conductive film 1106 which will eventuallyserve as a gate electrode, and a silicon oxide film 1107.

Next, as shown in FIG. 11(b), photoresist pattern 1108 is formed onsilicon oxide film 1107 by using a standard photomasking technique. Withthis pattern as a mask, silicon oxide film 1107 is selectively etched toform post-etch oxide film 1107A.

Thereafter, as shown in FIG. 11(c), photoresist pattern 1107 is removed.Then, with silicon oxide film 1107 as a mask, electrically conductivefilm 1106 is selectively etched and a gate electrode is formed. Ionimplantation of impurity atoms (donor or acceptor as described above)forms source and drain areas 1110, 1111 self-aligned to post-etchsilicon oxide film 1107A. Next, the gate electrode 1109 is selectivelyetched so that it is small relative to post-etch silicon oxide film1107A, thus forming offset gate electrode 1109.

This is similar to Process Example #5, however, in Example #5, the gateelectrode is over-etched, with respect to the overlying oxide layer,prior to ion implantation (i.e. source/drain formation). In Example #6,the source/drain forming ion implant is done while the gate electrode issubstantially of the same dimension as the overlying oxide layer. Afterion implant, in Example #6, the gate electrode is further etched so asto produce a gate electrode offset from the source and drain.

Thereafter, as shown in FIG. 11(d), source and drain electrodes 1112,1113 which may be comprised of a metal, a transparentelectrically-conductive film (e.g. ITO), or the like, are connected tosource and drain areas 1110, 1111 respectively, according to the usualprocess steps as would be understood by those skilled in this art. Thus,a TFT of the present invention is completed.

PROCESS EXAMPLE #7

The TFT of the present invention can be fabricated with the processsteps illustrated by the cross-sectional views of FIGS. 12(a)-12(d).

FIG. 12(a) shows patterns 1202, 1203 which are comprised of a siliconthin film, such as poly-Si or a-Si, formed on an insulating substrate1201 of glass, quartz, sapphire or the like. Pattern 1204 comprised of asilicon thin film, such as poly-Si or a-Si, is disposed so as to be incontact with the top side of the two pattern areas 1202, 1203 so as toconnect the two areas. Next, formed in turn on all of the above are gateinsulation layer 1205 comprised of an insulator such as a silicon oxidefilm, and electrically conductive film 1206 which will eventually serveas a gate electrode.

Next, as shown in FIG. 12(b), photoresist pattern 1207 is formed onelectrically conductive film 1206 by using a standard photomaskingtechnique. With this pattern as a mask, electrically conductive film1206 is etched to form gate electrode 1208.

FIG. 12(c), shows source and drain regions 1209, 1210 formed,self-alignedly to gate electrode 1208, by ion implantation. Next, gateelectrode 1208 is selectively etched so as to be small, thus producingthe offset gate structure 1208A.

Thereafter, as shown in FIG. 12(d), source and drain electrodes 1211,1212 which are comprised of a metal, a transparentelectrically-conductive film (e.g. ITO), or the like, are connected tosource and drain areas 1210, 1211 respectively according to the usualprocess steps as would be understood by those skilled in this art. Thus,a TFT of the present invention is completed.

PROCESS EXAMPLE #8

The TFT of the present invention can be fabricated with the processsteps illustrated by the cross-sectional views of FIGS. 13(a)-13(d).

FIG. 13(a) shows patterns 1302, 1303 which are comprised of a siliconthin film, such as poly-Si or a-Si, formed on an insulating substrate1301 of glass, quartz, sapphire or the like. Pattern 1304 comprised of asilicon thin film, such as poly-Si or a-Si, is disposed so as to be incontact with the top side of the two patterns 1302, 1303 so as toconnect the two patterns. Next, formed in turn on all of the above, aregate insulation layer 1305 comprised of an insulator such as a siliconoxide film, electrically conductive film 1306 which will eventuallyserve as a gate electrode, and silicon oxide film 1307.

Next, as shown in FIG. 13(b), photoresist pattern 1308 is formed onsilicon oxide film 1307 by using a standard photomasking technique. Withthis photoresist pattern as a mask, silicon oxide film 1307 isselectively etched to form post-etch oxide pattern 1307A.

Then, as shown in FIG. 13(c), with silicon oxide film 1307A as a mask,electrically conductive film 1306 is selectively etched and gateelectrode 1309 is formed. Thereafter, photoresist pattern 1308 isremoved. Then insulation layer 1310, which is preferably a silicon oxidefilm, or the like, is formed on all of the above. Then silicon oxidefilm 1310 is anisotropically etched so that it remains on the side wallsof gate electrode 1309. At this point in the process, gate electrode1309 is coated with the silicon oxide films 1307 and 1310. Ionimplantation of impurity atoms (donor or acceptor as described above)forms source and drain areas 1311, 1312 self-aligned to the sidewallplus gate electrode structure just described.

Thereafter, as shown in FIG. 13(d), source and drain electrodes 1313,1314 which are comprised of a metal, a transparentelectrically-conductive film (e.g. ITO), or the like, are connected tosource and drain areas 1311, 1312 respectively, according to the usualprocess steps as would be understood by those skilled in this art. Thus,a TFT of the present invention is completed.

PROCESS EXAMPLE #9

The TFT of the present invention can be fabricated with the processsteps illustrated by the cross-sectional views of FIGS. 14(a)-14(c).

FIG. 14(a) shows pattern 1402 which is comprised of a silicon thin film,such as poly-Si or a-Si, disposed on an insulating substrate 1401 ofglass, quartz, sapphire or the like. Next, all of the above is coatedwith gate insulation layer 1403 comprised of an insulator such as asilicon oxide film. Formed thereon, is gate electrode 1404 which may becomprised of a metal, an electrically conductive film, a poly-Si film towhich an impurity is added, or the like.

Next, as shown in FIG. 14(b), insulation layer 1405 is formed on all ofthe above. Insulation layer 1405 is preferably an oxide of silicon.Next, insulation layer 1405 and gate insulation layer 1403 areselectively etched so that at least a portion of pattern 1402 isexposed. Next, patterns 1406 and 1407 which are comprised of a poly-Sifilm, in which an impurity is added, are formed so as to connect topattern 1402. These patterns act as source/drain regions 1406, 1407,respectively.

Thereafter, as shown in FIG. 14(c), source and drain electrodes 1408,1409 which are comprised of a metal, a transparentelectrically-conductive film (e.g. ITO), or the like, are connected tosource and drain areas 1406, 1407 respectively, according to the usualprocess steps as would be understood by those skilled in this art. Thus,a TFT of the present invention is completed.

PROCESS EXAMPLE #10

The TFT of the present invention can be fabricated with the processsteps illustrated by the cross-sectional views of FIGS. 15(a)-15(c).

FIG. 15(a) shows patterns 1502, 1503 which are comprised of a siliconthin film, such as poly-Si or a-Si, formed on an insulating substrate1501 of glass, quartz, sapphire or the like. Pattern 1504 comprises asilicon thin film, such as poly-Si or a-Si, disposed so as to be incontact with the top side of the two patterns 1502, 1503. Next, all ofthe above is coated with gate insulation layer 1505 which is comprisedof an insulator such as a silicon oxide film. Formed thereon is gateelectrode 1506 comprised of a metal, an electrically conductive film, apoly-Si layer in which an impurity is added, or the like.

Then, as shown in FIG. 15(b), insulation layer 1507 comprising a siliconoxide film, or the like, is formed on all of the above. Next, aphotoresist pattern 1508 is formed thereon by using a standardphotomasking technique. With this pattern as a mask, ion implantation ofimpurity atoms (donor or acceptor as described above) forms source anddrain areas 1509, 1510 self-aligned to photoresist pattern 1508.

Thereafter, as shown in FIG. 15(c), photoresist pattern 1508 is removed.Source and drain electrodes 1511, 1512 which are comprised of a metal, atransparent electrically-conductive film (e.g. ITO), or the like areconnected to source and drain areas 1509, 1510 respectively according tothe usual process steps as would be understood by those skilled in thisart. Thus, a TFT of the present invention is completed.

CONCLUSION

As set forth above, the present invention can be used for manyapplications in which thin film transistors are used, for example, imagesensors or liquid crystal displays. The present invention makes a greatcontribution toward improving the performance of these devices whilereducing the costs thereof.

It should also be noted that the thin film which is used to form thechannel region in accordance with this invention may be recrystallizedor not.

With respect to polycrystalline silicon films it has been found thatgenerally, the thinner the film is made the better are the TFTproperties. However, it has been found that it is only down to about 100Å that polycrystalline silicon films behave as polycrystalline silicon,and that as these films become thinner they physically behave asamorphous silicon. In consideration of the physical changes that takeplace at about 100 Å thickness and the controllability of film thicknessduring processing, 250 Å film thickness has been chosen for typicalimplementations of the present invention.

With respect to recrystallized silicon films, crystal grain size is animportant parameter in determining TFT properties. Generally, with otherparameters held constant, bigger grain size gives better TFT properties.In a recrystallization process it is easier to achieve larger grain sizeas the starting film thickness is increased. Therefore the determinationof film thickness is made by considering the trade-offs between thickerfilms (i.e. larger grain size) and thinner films (better transistorcharacteristics). In typical implementations of the present invention,250 Å film thickness is chosen even if the film is to be recrystallized.

The film thicknesses typically used for the layers described in thevarious embodiments are approximately 250 Å for the channel region film,approximately 1500 Å for the initial pattern films, betweenapproximately 500 Å and 4,000 Å for the gate electrode film,approximately 8,000 Å for the source and drain contact materials, andbetween approximately 1,000 Å and 2,000 Å for the gate dielectric layer.

The present invention can be realized by using materials other thanthose mentioned above without departing from the spirit and scope of thepresent invention. Various embodiments are explained primarily byshowing a structure in which the thickness of silicon films of thesource and drain areas are different from that of the channel section.However, as shown in FIG. 16, a thin film transistor having a structurein which the thickness of silicon films of, for example, source area1601 and drain area 1602, are the same as that of channel area 1603, maybe used within the spirit and scope of the present invention.

While the invention has been described in conjunction with severalspecific embodiments, it is evident to those skilled in the art thatmany further alternatives, modifications and variations will be apparentin light of the foregoing description. Thus, the invention describedherein is intended to embrace all such alternatives, modifications,applications and variations as may fall within the spirit and scope ofthe subjoined claims.

What is claimed is:
 1. A thin film transistor, comprising:a) a sourcearea and an opposing drain area, having top sides, which are comprisedof a first thin film having a first concentration of impurity atoms anda first portion of a second thin film, disposed on said source and drainarea top sides and having a second concentration of impurity atoms; b) achannel area comprised of a second portion of said second thin filmformed between said source area and said drain area, wherein said secondportion of said second thin film is in contact with said source anddrain areas, said second portion of said second thin film having a thirdconcentration of impurity atoms; c) a gate insulation layer formed tocover at least said channel area; and d) a gate electrode disposed onsaid gate insulation layer;wherein said first concentration is greaterthan said second concentration, and said first concentration and saidsecond concentration are each greater than said third concentration. 2.The thin film transistor of claim 1, wherein said gate electrode islaterally offset from said source area and is laterally offset from saiddrain area.
 3. The thin film transistor of claim 1, wherein said gateelectrode is laterally offset from said drain area.
 4. The thin filmtransistor of claim 1, wherein said first thin film and said second thinfilm are silicon thin films.
 5. The thin film transistor of claim 1,wherein said impurity atoms are of the donor type.
 6. The thin filmtransistor of claim 1, wherein said impurity atoms are of the acceptortype.
 7. The thin film transistor of claim 1, wherein said gateelectrode is comprised of poly-Si.
 8. The thin film transistor of claim1, wherein said gate electrode is comprised of a transparentelectrically conductive material.
 9. The thin film transistor of claim1, wherein said gate insulating layer is an oxide of silicon.
 10. Thethin film transistor of claim 1, wherein said second thin film isrecrystallized.
 11. The thin film transistor of claim 1, wherein saidthird concentration is substantially zero.